Small AC oscillating motors may be used in handheld body care devices. Particularly, these motors may be used in body care devices ranging from beauty care devices that treat wrinkles by gently massaging the face to devices sexually stimulate persons experiencing sexual dysfunction.
Conventional AC oscillating motors are typically powered through main AC electrical services in order to draw sufficient electrical power. To increase the mechanical power output, conventional AC motors may use asymmetrical magnetic reluctance between the stator and the rotor. With asymmetrical magnetic reluctance, however, the rotor may rotate to an undesired position and may become magnetically locked at the undesired position when the power to it is turned off. To compensate for this undesirable rotor locking, conventional AC motors use a recall spring. The recall spring forces the rotor to a neutral position (or unlocked position) when the power is turned off.
To optimize the power consumed by conventional AC motors using asymmetrical magnetic reluctance and a recall spring, the motors must be tuned. A conventional motor may be optimally tuned when its natural resonance frequency is adjusted to be approximately equal to the frequency of the AC power source feeding the motor. Factors contributing to the natural resonance frequency include the recall spring's inertial constant and the rotor's moment of inertia. A conventional motor may be tuned, for example, by adjusting the rotor's moment of inertia by adding a specifically weighted flywheel to the rotor's shaft. The flywheel should be sufficiently weighted to give the conventional motor a natural resonance frequency approximately equal to the frequency of the motor's AC power source.
Because optimal turning is a function of the rotor's inertia, it logically follows than an instrument fixed on the rotor's shaft contributes to the rotor's inertia and thus to the motor's efficiency. In other words, the conventional motor can be tuned to its optimal point for only one power source frequency, one spring constant, one flywheel weight, and one instrument weight. Because it is not practical to change the spring and flywheel, the conventional motor can only give optimal performance with only one instrument mass driven by the rotor's shaft. Moreover, conventional motors are optimally tuned to only one power source frequency. Therefore, a conventional motor tuned for a 50 Hz power source, for example, would no longer be optimally tuned when powered by a 60 Hz source.
This turning problem may not be a major concern for devices using the same type of instrument, such as, for example, electric toothbrushes having stem-brushes of the same shape, weight, and dimensions. However, this problem may become a concern when several types of instruments may be used, with each having a different shape, weight, and rotational inertia. With a different type mass attached to the rotor's shaft, the motor's tuning is thus no longer optimal. This may be particularly true for body massaging devices, such as sexual stimulation devices that use a variety of differently shaped and weighted instruments.
Furthermore, the tuning problem occurs when a device is tuned for one power source frequency and is subsequently operated at another power source frequency. For example, an electric tooth brush optimally tuned for a 50 Hz power system would no longer be optimally tuned if operated on a 60 Hz system, even if the same instrument is placed on the tooth brush's shaft on both power systems.
The aforementioned tuning problem becomes worse when the instrument using the conventional motor has an applied load (e.g., when it is pressed against a body part). In this case, the mechanical frequency of the rotor system increases and is no longer tuned properly. When this happens, the mechanical power at the rotor's shaft decreases rapidly to a very small value because the natural resonance frequency becomes higher than the power source frequency.